Green’s Dictionary of Slang

a mixture of two sorts of food or drink.

1813		Examiner (London) 17 May 317/1: That injudicious mixing of wines, which is called matrimony .
1883		Ogilvie’s Imperial Dict. (new edn) n.p.: Matrimony, [...] 4. A name given jocularly to raisins and almonds mixed, and various other common combinations .
1892		M. North Recollections Happy Life I 103: They gave us glasses of ‘matrimony’, a delicious compound made of star-apple sugar and the juice of Seville oranges .
1920		G. Saintsbury Notes on a Cellar-Book 90: It looked and tasted rather like the curious compound called ‘matrimony’ [...] Few decent middle-class households then were without their standing decanters of port and sherry, which, when the port was getting rather stale, were mixed together as remnants, and so nothing was lost.